Equalizer



Patented June 22, 1954 res n? EQUALIZER Stephen Bobis and Walter R. Lundry, Summit,

N. J., assignors to Bell Telephone Laboratories,

Incorporated, New York,

of New York N. Y., a corporation Application September 8, 1950, Serial No. 183,872

1 Claim. i

This invention relates to wave transmission networks and more particularly to adjustable multirange loss equalizers.

The principal object of the invention is to adjust the loss in a wave transmission system independently over a number of adjoining or overlapping frequency ranges.

Further objects are to reduce the fiat loss, the number of component elements and the cost, and to simplify the construction, of an adjustable multirange loss equalizer.

In the embodiments disclosed herein the adjustable multirange loss equalizer in accordance with the present invention is adapted for operation between a source of alternating signals and a load to introduce an adjustable amount of loss in each of at least three overlapping frequency ranges. The loss may be either positive or negative relative to a constant, or fiat, loss. The equalizer comprises at least three like impedance arms connected in like manner to form a coupling branch between the source and the load. Each arm is resonant at a different frequency, corresponding approximately to the maximum adjustable positive loss in one of the ranges, and comprises an inductance, a capacitance and an adjustabie resistance. The inductance and capacitance in one of the arms have values determined by the impedance of the source, the impedance of the load, the resonant frequencies of the three arms, a factor chosen to make the loss of the equalizer substantially constant over all of the ranges for predetermined equal normal settings or the resistances, and the current ratio corresponding to this constant loss.

Four different circuit arrangements of the component impedance elements of the equalizer are shown. In two the inductance, capacitance and resistance in each arm are connected in series, the arms connected in parallel to form the coupling branch, and the coupling branch connected either in parallel or series with the source and the load. In the other two, the elements in each arm are connected in parallel and the arms are connected in series to form a coupling branch which is connected either in series or in parallel with the source and the load. Additional impedance elements may be included in order to improve the performance of the equalizer, especially over the highest and the lowest frequency ranges.

Compared with known types of adjustable multirange loss equalizers, the ones disclosed herein require fewer elements, cost less to manufacture, and have less flat loss. They are particularly useful where a large number of small loss adjustments are required, as in mop-up equalizers for television systems, but may also be used for larger but perhaps less precise adjustments.

The nature of the invention will be more fully understood from the following detailed description and by reference to the accompanying drawings, of which:

Figs. 1 and 2 are schematic circuits of adjustable equalizers in accordance with the invention in which the elements in each impedance arm are connected in series and the arms are connected. in parallel to form a coupling branch which is connected in parallel or in series with the source and the load;

Figs. 3 and 4 show two other embodiments of the invention in which the elements in each arm are connected in parallel, the arms are connected in series to constitute the coupling branch, and the coupling branch is connected in series or in parallel with the source and the load; and

Fig. 5 shows typical insertion loss-frequency characteristics obtainable with the equalizer of Fig. 1.

Taking up the figures in greater detail, the circuit of Fig. 1 shows one embodiment of an adjustable multirange loss equalizer in accordance with the invention comprising a pair of input terminals 10, II and a pair of output terminals l2, I3. A signal source of alternating electromotive force M of impedance Rs is connected to the input terminals It, II and a suitable load of impedance R1. is connected to the output terminals I2, 13.

The equalizer comprises at least three like impedance arms I6, I! and it connected in like manner to form a coupling branch between the source impedance Rs and the load R1,. The equalizer will introduce an adjustable amount of loss, either positive or negative relative to the fiat loss, in each of three frequency ranges, with maximum adjustable positive loss at the frequencies 13-1, f1 and n+1, respectively, More frequency ranges may be provided by adding more impedance arms similar to l6, I! or it, as indicated by the broken lines [9. An equalizer with more than fifty such arms, operable over a like number of frequency ranges, has been made and successfully tested.

In Fig. 1 each of the arms l6, ll and I 8 comprises an inductance, a capacitance and a variable resistance connected in series. In arm I! these elements are, respectively, Li, C1, and R1, in arm [6 they are Ll-1, (31-1 and Rl-l, and. in

arm l8 they are Ll+l, C1+1 and Ri+1. The arms l6, l1 and iii are connected in parallel to form a coupling branch which is connected in parallel with the source impedance Rs and the load R1,. Each of the arms l6, l1 and i8 is resonant at a different frequency corresponding approximately to the maximum adjustable positive loss in one of the frequency ranges.

The values of the inductance L1 and the capacitance C1 in arm [1 are determined by the source impedance Rs, the impedance of the load R2, the three frequencies f1, n+1 and f1 1, a factor K chosen to make the loss of the equalizer substantially constant over all of the ranges for predetermined equal normal settings of the variable resistances R1, R1+1 and R1 1, and the current ratio A corresponding to this constant loss. Explicitly, the elements Li and C1 in arm 11 have approximately the following values:

and the optimum value of K falls between two and three. Similar expressions may be set up for the values of the inductance and capacitance in each of the other arms It and It. The minimum average value of the nominally constant flat loss is determined by the adjustment range required. The flat loss will ordinarily have small deviations from its average value. These deviations are, in general, reduced to tolerable amplitudes by sufficiently increasing the fiat loss. For the range of K given, the normal value of each of the variable resistances will fall between 0.5 R01 and 1.5 R01.

It should be pointed out that the effect of the inclusion in Equations 1 and 2 of the frequencies n+1 and f1 1 of the neighboring arms of the equalizer and the term R01 is to take advantage of the contributions of those neighboring arms to the total conductance and thus the total loss of the equalizer arm for the frequency ,fi and the intermediate frequencies. It is an aspect of this invention that the interactions between neighboring arms of the equalizer are utilized to attain substantially flat normal conductance and loss characteristics for equal normal settings of the variable resistances R1, R1+1, and R1 1 due to the elements L1 and C1 having the particular values determined by the Formulas 1, 2 and 3.

The two shunt impedance arms Z11 and Z12 are added, when required, to improve the loss characteristic, especially over the highest and the lowest frequency ranges, or to provide some special type of correction. They may include one or more inductances, capacitances or resistances and are designed to meet the partcular loss requirements of the equalizer.

Fig. gives typical insertion loss-frequency characteristics obtainable with an equalizer of the type shown in Fig. 1. The loss in decibels relative to the average flat loss, shown by the horizontal line I5, is plotted against the frequency in megacycles. That is, the departure, either positive or negative, from the flat loss is shown. The solid-line curves 2!! and 2i! define the adjustment limits obtainable by varying the resistance R1-1 while keeping the resistances R1 and R1+1 at their normal values. The brokenline curves 2|, 2| and the dot-and-dash curves 22, 22' give the same limits for adjustments, respectively, of the resistances R1 and Ri+l with the other variable resistances set at their normal value. As already mentioned, the normal values for all of the resistances R1, R1+1 and Ri-l required to give a fiat loss are approximately equal. It will be noted that the frequency at which the maximum positive loss occurs in each range substantially coincides with the frequency of the maximum negative loss. The spacing of these maxima is dictated by the type of loss correction required. The pattern may, for example, be arithmetic, geometric, a combination of these, or of some other type. In the characteristics shown, each of the three positive loss curves 20, 2| and 22 overlaps the adjacent curve by at least half, and each of the negative curves 20, 2| and 22 overlaps the adjacent one by more than half.

Fig. 2 shows a second embodiment of the invention. The coupling branch is made up of the parallel combination of three arms 23, 24 and 25 and the impedances Z21 and Z22 which correspond, respectively, to the arms l1, l8, l9 and the impedances Z11 and Z12 in Fig. 1. In Fig. 2, however, the coupling branch is connected in series with the source impedance Rs and the load RL.

A third embodiment of the invention is shown in Fig. 3. The coupling branch comprises the three arms 21, 2B and 29 and the two impedances Z31 and Z12 all connected in series with the source impedance Rs and the load R1,. Each of the arms 21, 28 and 29 consists of the parallel combination of an inductance, a capacitance and a variable resistance. In arm 28 these elements are, respectively, L C and R in arm 21 they are L1-1, C1' 1 and R1-1, and in arm 29 they are L +1, C1+1 and R +1. The impedances Z31 and Z32 are included for the same purpose as the impedances Z11 and Z12 in Fig. 1. The arms 21, 28 and 29 are antiresonant, respectively, at the frequencies f1-1, f1 and n+1. The elements in the arm 28 have approximately the following values:

A has the same significance as in Equation 3, and K again has an optimum value falling between two and there to obtain a substantially fiat loss over all of the ranges. For this range of K the normal value of each of the variable resistances required to give a flat loss falls between Rea/0.5, or 2Ro2, and Roz/1.5, or 0.67 R02. Similar expressions may be written for the inductance and capacitance in each of the other arms 21 and 29.

Fig. 4 shows a fourth embodiment of the invention. The coupling branch comprises the series combination of the arms 31, 32 and 33 and the impedances Z41 and Z42 which correspond, respectively, to the arms 21, 28, 29 and the impedances Z31 and Zeain Fig. 3. However, in this.

case, the coupling branch is connected in parallel with the source impedance Rs and the load RI...

By proper design. loss characteristics similar to those given in Fig. 5 may be obtained with any one of the circuits shown in Figs. 2, 3 and 4. The choice of circuit depends, among other things, upon the magnitudes of the source impedance Rs and the load impedance R1. and upon the required values of the component elements. It will be understood, of course, that additional impedance arms may be employed in any of these circuits to provide additional adjustment ranges, if desired.

What is claimed is:

An equalizer for operation between a signal source of impedance Rs and a load of impedance RL to introduce an adjustable amount of loss in each of at least three adjoining frequency ranges comprising at least three impedance arms connected directly in parallel with both said source and said load, each of said arms comprising the series combination of an inductance, a capacitance and an adjustable resistance, and the inductance Li and capacitance C1 in one of said arms having approximately the values References Cited in the file of this patent UNITED STATES PATENTS where Number Name Date 1,693,401 Nyman Nov. 27, 1928 2,182,328 Weinberger Dec. 5, 1939 2,550,595 Pfieger Apr. 24, 1951 FOREIGN PATENTS Number Country Date 688,979 France May 20, 1930 

